Connected: An Internet Encyclopedia
RFC 822 - 2. NOTATIONAL CONVENTIONS

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### 2. NOTATIONAL CONVENTIONS

This specification uses an augmented Backus-Naur Form (BNF) notation. The differences from standard BNF involve naming rules and indicating repetition and "local" alternatives.

#### 2.1. RULE NAMING

Angle brackets ("<", ">") are not used, in general. The name of a rule is simply the name itself, rather than "<name>". Quotation-marks enclose literal text (which may be upper and/or lower case). Certain basic rules are in uppercase, such as SPACE, TAB, CRLF, DIGIT, ALPHA, etc. Angle brackets are used in rule definitions, and in the rest of this document, whenever their presence will facilitate discerning the use of rule names.

#### 2.2. RULE1 / RULE2: ALTERNATIVES

Elements separated by slash ("/") are alternatives. Therefore "foo / bar" will accept foo or bar.

#### 2.3. (RULE1 RULE2): LOCAL ALTERNATIVES

Elements enclosed in parentheses are treated as a single element. Thus, "(elem (foo / bar) elem)" allows the token sequences "elem foo elem" and "elem bar elem".

#### 2.4. *RULE: REPETITION

The character "*" preceding an element indicates repetition. The full form is:

                                  <l>*<m>element

indicating at least <l> and at most <m> occurrences of element. Default values are 0 and infinity so that "*(element)" allows any number, including zero; "1*element" requires at least one; and "1*2element" allows one or two.

#### 2.5. [RULE]: OPTIONAL

Square brackets enclose optional elements; "[foo bar]" is equivalent to "*1(foo bar)".

#### 2.6. NRULE: SPECIFIC REPETITION

"<n>(element)" is equivalent to "<n>*<n>(element)"; that is, exactly <n> occurrences of (element). Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three alphabetic characters.

#### 2.7. #RULE: LISTS

A construct "#" is defined, similar to "*", as follows:

                                  <l>#<m>element

indicating at least <l> and at most <m> elements, each separated by one or more commas (","). This makes the usual form of lists very easy; a rule such as '(element *("," element))' can be shown as "1#element". Wherever this construct is used, null elements are allowed, but do not contribute to the count of elements present. That is, "(element),,(element)" is permitted, but counts as only two elements. Therefore, where at least one element is required, at least one non-null element must be present. Default values are 0 and infinity so that "#(element)" allows any number, including zero; "1#element" requires at least one; and "1#2element" allows one or two.

#### 2.8. ; COMMENTS

A semi-colon, set off some distance to the right of rule text, starts a comment that continues to the end of line. This is a simple way of including useful notes in parallel with the specifications.

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Connected: An Internet Encyclopedia
RFC 822 - 2. NOTATIONAL CONVENTIONS